In an image reading apparatus such as a microfilm reader connected to an external device such as a personal computer, read image data are temporarily stored in the internal buffer memory of the image reading apparatus, sequentially transferred to the external device such as a personal computer via an interconnection, and stored or displayed.
In the prior art, the buffer memory must have a capacity of at least one frame in order to output image data to an external device such as a personal computer while scanning the image, or a capacity of one frame or more in order to successively scan a plurality of images in consideration of a possibility that image data which has not been transferred remains in the buffer memory.
As the CPU speeds of recent personal computers or the like increase, image data can be transmitted to an external device such as a personal computer without any buffer memory while the image is scanned. The same effects can also be obtained even if the buffer memory does not have a sufficient capacity in an arrangement in which image data are temporarily stored in the internal buffer memory of an image reading apparatus and sequentially output from the buffer memory to an external device such as a personal computer.
While an image is scanned, the image data are sequentially transmitted to an external device such as a personal computer. This can shorten the time taken from the start of scan to the end of image data transfer, can realize successive scan, and can reduce the cost.
However, the data reception speed of an external device such as a personal computer may decrease owing to internal processing of the external device or processing of another device connected to the external device, and may become lower than the image reading speed of the image reading apparatus. In this case, any measure must be taken when the capacity of the buffer memory is insufficient.
As a measure, an image reading apparatus constituted to move a scan unit such as a general flat bed scanner stops movement of the scan unit to temporarily stop scan, waits until data are satisfactorily transferred from the image reading apparatus, and then restarts scan from the same position. Alternatively, the scan unit is moved at a low speed or intermittently moved to make the image data transmission speed catch up with the data reception speed. A sheet through scanner also obtains the same effects by controlling the sheet convey speed.
If this operation is done in an image reading apparatus such as a microfilm reader having an enlargement/projection system, vibrations generated by accelerating/decelerating movement of the scan unit transmit to an optical system and typically generate an image distortion.
That is, the moving speed of the scan unit is controlled by controlling the rotational speed of the motor. The moving speed of the scan unit varies due to variations in the physical inertia of the motor and belt in accelerating/decelerating the motor. As a result, a read image distorts. As for stopping of rotation of the motor, an image may distort due to the same cause upon accelerating the motor at the restart of movement.
If the motor vibrates upon changes in the rotational speed of the motor or variations in load on the motor, the vibrations are transmitted to the optical system to distort a display image and resultantly distort a scanned image.